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Preprint  of  Copyrighted  Proceedings 
AMERICAN  CONCRETE  INSTITUTE 

Subject  to  Revision 
Not  Released  for  Publication 


AMERICAN  CONCRETE  INSTITUTE 

The  following  paper  is  to  be  presented  at  the  annual  con- 
vention of  the  Institute,  Chicago,  February  14  to  16,  1921.  Written 
discussion  is  solicited  and  should  be  forwarded  to  Harvey  Whipple, 
Secretary,  American  Concrete  Institute,  New  Telegraph  Building, 
Detroit,  Mich. 


TEST  OF  A FLAT-SLAB  FLOOR  OF  THE  NEW  CHANNON  BUILDING 
By  H.  Ff^ONNERMAN*  AND  F.  E.  RlCHART** 

II  *» 

It  is  the  object  of  this  paper  to  present  the  results  of  a test  made  by 
the  writers  in  July,  1920,  upon  a floor  slab  of  the  new  Henry  Channon 
Building  in  Chicago.  The  test  was  made  under  the  general  direction  of  a 
commission  appointed  by  the  Chicago  Building  Department  and  consisting 
of  Prof.  A.  N.  Talbot,  of  the  University  of  Illinois;  Prof.  D.  A.  Abrams, 
of  Lewis  Institute,  and  B.  E.  Winslow,  of  the  Chicago  Building  Depart- 
ment. The  architect  for  the  building  was  A.  S.  Alschuler,  the  designing 
engineers  Morrison  and  Beck,  and  the  contractor  R.  F.  Wilson  & Co.,  all 
of  Chicago. 

The  building  in  which  the  test  was  made  is  a seven-story  reinforced- 
concrete  structure  located  at  the  southeast  corner  of  Market  and  Randolph 
Sts.,  Chicago,  Illinois,  and  was  erected  during  the  summer  of  1920.  The 
type  of  floor  construction  used  throughout  the  building  was  a modified  form 
of  the  Smulski  or  S-M-I  System;  the  principal  variation  from  the  standard 
S-M-I  design  being  in  the  arrangement  of  rectangular  and  diagonal  bands 
over  the  column  heads,  and  in  the  distribution  and  amount  of  steel  in  the 
different  ring  units. 

The  location  of  the  test  panels  in  the  building  is  shown  in  Fig.  1. 

General  information  regarding  the  floor  and  the  test  is  summarized 
below  for  convenient  reference. 


Slab. 


General  Data  of  Slab  and  Test. 

Area  loaded  4 interior  panels,  4th  floor. 

Panel  dimensions  20  ft.  y2  in.  square. 

Nominal  thickness  of  slab 8 in. 

Nominal  thickness  at  drop  panel 12*4  in. 

Dimensions  of  drop  panel 6 ft.  6 in.  square. 

Diameter  of  column  capitol 4 ft.  6 in. 

Age  of  slab  at  beginning  of  load  test ...  53  days. 


♦Los  Angeles,  California. 

♦♦University  of  Illinois,  Urbana,  Illinois. 


: 


2 


Flat-Slab  Floor  Test. 


Loads. 


Design  load  

Dead  load  

Loading  material  for  test 


200 

lb. 

350 

lb. 

Load  increments  and 

500 

lb. 

length  of  time 

650 

lb. 

applied. 

500 

lb. 

500 

to  0 

/SO -S' 

200  lb.  per  sq.  ft. 

100  lb.  per  sq.  ft. 

Building  brick. 

per  sq.  ft.  for  about  24  hours, 

per  sq.  ft.  for  about  24  hours, 

per  sq.  ft.  for  about  24  hours, 

per  sq.  ft.  for  about  18  hours, 

per  sq.  ft.  for  about  1 week, 

per  sq.  ft.  for  about  2 weeks. 


Afaz-A-ef  S/ 


FIG.  1.  LOCATION  OF  TEST  PANELS. 


Observations. 

Number  of  strain  gage  lines 337 

Number  of  deflection  points 22 

Number  of  observations  made 4000 


Arrangement  of  Slab  Reinforcement. 

The  reinforcement  in  the  test  floor  consisted  of  rings,  straight  bars 
and  “truss  bars.”  The  general  arrangement  of  the  reinforcing  bars  in  the 
test  area  as  designed,  is  shown  in  Fig.  2.  The  size  of  all  bars  was  checked 
before  the  floor  was  poured  and,  with  a few  slight  exceptions  the  rein- 
forcement was  as  shown  in  Fig.  2.  The  position  of  the  reinforcement  was 
also  inspected  before  pouring  and  in  general  the  steel  was  found  to  be 


Flat-Slab  Floor  Test. 


3 


Schedule  of  Reinforcement 
Fourth  F/oor  S/ab  -Live  Load -200 /h per  sq  ft Co/umns 


Unit 

Truss 

Rods 

•Sfmigh 

Rods 

Rings  - size  and  diameter 

5tory 

Column  No. 

3 '-O' 

4-6" 

5'-0"\  6-0' 

7-6 ' 

9'-0' 

/0-6' 

/210‘ 

/3'-3" 

!4'-6‘ 

D78  £7  £678 

06  £ 6,8 

A 

3-ft 

ft 

ft 

ft 

ft 

ft 

ft 

4 

Co/  ze  '4 
Core  22" 0 
tods  //-/"<* 
Spiral  ia-Zp 

28’ 0 
24’  0 
/6-fo 
10-/ ZP 

B 

2-i'° 

i-r+ 

4 t 

ft 

ft 

ft 

ft 

ft 

ft 

f* 

ft 

c 

4-}  V 

l* 

ft 

ft 

ft 

3 

Co/  28“  0 
Core  24" f 
Tods  //-/}> 
Spiral  ,i"0-2i‘p 

30  "a 

26"  4 
/'8-fa, 
f*;2"P 

T 

II-}  4 

FIG.  2.  DETAILS  OK  REINFORCEMENT. 


4 


Flat-Slab  Floor  Test. 


reasonably  close  to  its  designed  position  in  plan.  In  pouring  the  concrete 
the  steel  became  displaced  vertically  in  many  places.  Care  was  taken  when 
placing  the  ring  units  in  the  test  area  to  avoid  lapping  of  the  bars  at 
places  where  gage  lines  were  to  be  located,  but  in  one  instance  a gage 
line  was  located  where  a lap  occurred.  The  bars  of  the  ring  units  were 
generally  lapped  about  40  diameters,  and  the  position  of  laps  is  shown  in 
Fig.  2.  A view  of  a part  of  the  test  area  just  before  the  concrete  was 
poured  is  shown  in  Fig.  3.  A large  number  of  corks  were  placed  between 
the  reinforcing  bars  and  the  form  to  facilitate  the  opening  of  gage  lines 
in  the  steel,  and  some  small  steel  plugs  were  lightly  tacked  to  the  forms 


FIG.  3.  VIEW  OF  REINFORCEMENT  IN  TEST  AREA. 


to  provide  for  gage  points  for  measuring  compressive  strains  in  the  con- 
crete. Corks  were  also  wired  to  bars  near  the  upper  surface  of  the  slab, 
and,  although  they  were  usually  covered  up  during  the  pouring  of  the 
concrete,  they  were  quite  useful  in  aiding  the  location  of  gage  lines  on 
the  reinforcing  bars. 

Havemeyer  deformed  bars  were  used  for  reinforcement  with  the  excep- 
tion of  a few  plain  round  bars  in  the  ring  units.  There  was  not  much 
variation  in  the  strength  of  the  various  sizes  of  bar,  as  found  from  tension 
tests  on  coupons  cut  from  the  reinforcement  as  it  was  being  placed.  The 
average  unit  stress  at  the  yield  point  was  found  to  be  50,000  lb.  per  sq.  in., 
and  the  average  ultimate  strength  84,000  lb.  per  sq.  in. 


Flat-Slab  Floor  Test. 


5 


Concrete  in  Test  Floor. 

The  concrete  in  the  test  panels  was  poured  May  7,  1920.  It  was  mixed 
in  the  proportion  of  1 part  Universal  portland  cement,  2 parts  torpedo 
sand  and  4 parts  broken  limestone  and  was  of  quite  uniform  consistency. 
The  concrete  was  poured  rather  wet,  excess  water  being  taken  up  by  scat- 
tering a mixture  of  cement  and  sand  over  the  surface  and  then  finishing 
with  trowels.  A monolithic  floor  finish  was  secured  by  troweling  into  the 
surface  of  the  floor  a mixture  of  equal  parts  of  cement  and  ironite. 

During  the  pouring  of  the  floor,  6 x 12-in.  test  cylinders  were  made 
of  concrete  from  various  parts  of  the  test  area.  This  concrete  gave  a 
slump  of  8 to  9 in.  in  a 6 x 12-in.  cylinder.  The  results  of  compression 
tests  made  on  these  cylinders  at  Lewis  Institute  are  given  in  Table  I. 


Table  I. — Compression  Tests  of  Concrete. 


No.  of 
Cylinders 
Tested 

5 

Manner  of 
Storage 

In  air 

Age  at 
Test 
Days 

7 

Compressive 
Strength 
lb.  per  sq.  in. 
1270 

Initial  Modulus 
of  Elasticity 
lb.  per  sq.  in. 
2,840,000 

5 

a cc 

28 

2280 

3,200,000 

6 

« u 

60 

2870 

3,630,000 

5 

a cc 

66 

3070 

3,660,000 

5 

In  moist  closet 

28 

2750 

4,110,000 

5 

a ((  (( 

60 

3800 

4,720,000 

The  Test. 

The  test  was  performed  in  the  ordinary  manner  by  applying  a load 
and  taking  observations  of  deformations  and  deflections  at  various  stages 
of  the  loading.  There  were  163  strain  gage  lines  on  the  reinforcing  steel 
and  174  on  the  concrete,  making  337  in  all.  Leadings  of  the  deflection  of 
the  test  floor  were  taken  at  22  points.  The  manner  of  taking  strain  read- 
ings is  well  known,  but  the  scheme  for  measuring  deflections  differed  from 
the  usual  methods.  As  shown  in  Fig.  5,  an  Ames  micrometer  dial  was 
mounted  on  a long  wooden  pole,  the  extreme  ends  of  the  pole  and  dial 
plunger  being  shod  with  conical  steel  points.  These  points  engaged  small 
holes,  drilled  in  steel  plates  attached  to  the  lower  side  of  the  test  slab  and 
the  floor  below.  The  one  instrument  was  carried  from  point  to  point  and 
was  frequently  checked  on  a standard  gage  length.  It  is  believed  that  the 
instrument  produced  very  reliable  results.  This  means  of  measuring  deflec- 
tion is  much  simpler  than  that  of  erecting  timber  standards  at  each  deflec- 
tion point,  and  is  less  liable  to  accidental  disturbance. 

The  loading  material  was  brick  which  later  was  used  in  the  construc- 
tion of  walls  of  the  building.  The  individual  brick  were  approximately 
2y8  x 3%  x 8 in.  in  size,  and  their  average  weight  was  4 lb.  From  meas- 
urements and  counts  of  the  brick  as  they  were  piled  on  the  test  panels  the 
weight  per  sq,  ft.  of  area  for  one  layer  of  brick  on  edge  was  found  to  be 


6 


Flat-Slab  Floor  Test. 


about  30  lb.,  and  this  value  was  used  in  calculating  the  various  increments 
of  load.  The  bricks  were  piled  on  the  test  panels  by  brick  masons  in  a 
workmanlike  manner,  and  it  is  believed  that  the  load  of  30  lb.  per  sq.  ft. 
closely  represents  the  actual  weight  per  sq.  ft.  applied  to  the  floor  for  one 
layer  of  brick.  In  piling  the  brick,  aisles  generally  not  over  4 in.  in  width 
were  left  as  indicated  in  Fig.  1,  in  order  to  prevent  arching  of  the  loading 
material.  The  space  covered  by  the  brick  amounted  to  96  per  cent  of  the 
total  area  of  the  four  loaded  panels. 

The  load  was  applied  in  four  increments,  one  day  being  required  to 
place  each  increment  of  load.-  The  load  at  which  strain  gage  readings  and 


FIG.  4.  VIEW  OF  TEST  LOAD. 


deflection  readings  were  taken  were  as  follows:  200  lb.  per  sq.  ft.  (6  layers 
of  brick  on  edge,  1 layer  on  side ) ; 350  lb.  per  sq.  ft.  (11  layers  on  edge, 
1 layer  on  side)  ; 500  lb.  per  sq.  ft.  (16  layers  on  edge,  1 layer  on  side)  ; 
650  lb.  per  sq.  ft.  (21  layers  on  edge,  1 layer  on  side).  The  maximum 
applied  load  was  two  and  one-sixth  times  the  design  live  and  dead  load; 
with  the  load  of  the  floor  included,  the  total  load  amounted  to  two  and 
one-half  times  the  design  live  and  dead  load.  Since  the  floor  was  already 
stressed  by  its  own  weight,  only  the  strains  produced  by  the  applied  load 
were  measurable. 

Small  tunnels  of  timbers  and  planking  were  built  over  the  gage  lines 
on  the  upper  surface  of  the  test  floor,  and  since  the  same  number  of  layers 


Flat-Slab  Floor  Test. 


7 


of  brick  was  piled  on  the  top  of  the  tunnels  as  elsewhere,  the  intensity  of 
load  on  these  tunnels  was  the  same  as  that  on  other  parts  of  the  test 
floor.  The  load  on  the  tunnels  was  transmitted  to  the  floor  in  such  a 
manner  as  to  cause  but  little  variation  in  either  moment  or  shear  from 
a condition  of  uniform  loading.  Fig.  4 gives  a view  of  a portion  of  panels 
A and  B showing  the  load  of  650  lb.  per  sq.  ft.  in  place  on  the  floor. 

Before  load  was  applied  to  the  test  panels  duplicate  sets  of  strain - 
gage  readings  and  of  deflection  readings  were  taken  on  all  gage  lines  and 
deflection  points.  One  set  of  strain  readings  was  taken  on  all  gage  lines 
at  a load  of  200  lb.  per  sq.  ft.,  and  at  the  other  loads  two  sets  of  strain 
readings  were  generally  taken  on  all  gage  lines,  the  second  set  of  readings 
being  taken  after  the  load  had  been  in  place  from  12  to  14  hours.  When 
the  second  set  of  strain  readings  at  a load  of  650  lb.  per  sq.  ft.  had  been 
taken  after  this  load  had  been  in  place  for  approximately  18  hours,  the 
last  increment  of  load  was  removed,  leaving  a load  of  500  lb.  per  sq.  ft. 
on  the  floor.  Deflection  readings  under  the  latter  load  were  then  taken. 
A load  of  approximately  500  lb.  per  sq.  ft.  remained  on  the  floor  for  7 
days,  when  strain  readings  and  deflection  readings  were  taken  on  all  gage 
lines  and  deflection  points.  During  the  following  20  days  the  bricks  were 
gradually  removed  from  the  test  floor  as  they  were  needed  in  the  con- 
struction of  the  building.  When  all  the  load  had  been  removed,  a complete 
set  of  strain  and  deflection  readings  was  taken. 

Readings  of  the  temperature  of  the  air  in  the  building  taken  from  time 
to  time  as  the  strain  readings  were  being  taken  ranged  from  71°  to  87°  F. 
over  the  period  of  the  test.  It  is  probable  that  the  variation  in  the  tem- 
perature of  the  floor  slab  was  much  less  than  the  variation  in  the  air 
temperature  and  strain  readings  taken  on  a reinforcing  Bar  in  the  fourth 
floor  of  the  building  well  away  from  the  loaded  area  and,  therefore,  un- 
stressed by  load,  were  in  no  case  more  than  one  division  on  the  dial  of 
the  strain  gage  away  from  the  average  of  the  readings,  a difference  which 
corresponds  to  750  lb.  per  sq.  in.  of  steel  stress.  As  the  usual  run  of 
differences  was  much  less  than  this  it  was  thought  not  necessary  to  make 
a correction  for  temperature  and,  accordingly,  none  was  made. 

Deflections. 

The  location  of  all  deflection  points  and  the  deflection  of  the  floor 
under  various  increments  of  load  are  shown  in  Fig.  5.  The  diagram 
shows  a remarkable  uniformity  of  action  at  corresponding  points  in  the 
slab.  The  maximum  deflection  under  the  load  of  650  lb.  per  sq.  ft.  is  seen 
to  be  0.59  in.  at  deflection  point  4,  while  the  average  of  the  maximum 
deflections  at  the  centers  of  the  four  panels  is  0.54  in.  The  increase  in 
deflection  as  each  load  increment  remained  on  the  floor  over  night  is  evi- 
dent from  the  diagram,  especially  at  the  higher  loads. 

The  amount  of  recovery  of  the  slab  toward  its  original  position  after 
all  load  had  been  removed  is  also  shown  in  the  diagram  by  circles  at  the 
line  of  zero  load. 


8 


Flat-Slab  Floor  Test, 


Flat-Slab  Floor  Test. 


9 


Appearance  of  Cracks. 

A study  of  the  appearance  of  the  cracks  in  the  test  slab  is  useful  not 
only  because  the  cracks  indicate  regions  of  high  tensile  stresses,  but  also 
because  the  size  and  distribution  of  the  cracks  furnish  a good  index  of 
the  relative  amount  of  tension  being  carried  by  the  concrete.  The  cracks 
on  the  lower  surface  of  the  sfUlb  were  first  observed  at  the  load  of  350  lb. 
per  sq.  ft.  at  the  center  of  the  loaded  panels  and  at  points  midway 
between  columns.  At  the  load  of  500  lb.  per  sq.  ft.  generally  a single 
crack  extended  along  the  section  of  maximum  positive  moment  from  center 
to  center  of  adjoining  loaded  panels.  At  the  load  of  650  lb.  per  sq.  ft. 
several  cracks  had  formed  as  shown  in  Fig.  6,  running  parallel  to  the 
cracks  first  noted  and  within  a narrow  zone  about  2 ft.  wide.  The  presence 


Load-  650  Lbpersqff 

FIG.  6.  CRACKS  ON  UPPER  AND  LOWER  SURFACES  OF  SLAB. 

of  such  cracks  constitutes  a good  criterion  of  the  spread  of  higher  stresses 
developed  by  the  positive  bending  moment.  It  has  been  observed  in  other 
tests  that  as  the  load  on  the  slab  approaches  the  maximum  which  the 
slab  will  carry,  cracks  are  formed  outside  the  belt  observed  here.  For  a 
greater  load  than  650  lb.  per  sq.  ft.,  then,  a wider  belt  of  cracks  would 
probably  be  visible,  and  a smaller  proportion  of  the  moment  would  be 
carried  by  the  tension  in  the  concrete.  Careful  search  failed  to  show  any 
cracks  following  the  direction  of  the  ring  units  in  the  bottom  of  the  slab, 
such  as  have  been  found  in  other  tests  at  higher  stresses. 

As  nearly  all  of  the  upper  surface  of  the  floor  was  covered  by  the 
loading  material,  the  development  of  cracks  during  loading  could  only  be 
followed  near  the  tension  gage  lines  in  the  observation  tunnels,  but  it 
was  undoubtedly  similar  to  that  on  the  lower  surface.  After  the  load  was 
removed  the  cracks  on  the  upper  surface  were  mapped,  but  it  is  likely 
that  many  of  the  finer  ones  had  closed  up,  leaving  only  the  larger  ones 


10 


Flat-Slab  Floor  Test. 


visible;  the  visible  cracks  are  shown  in  Fig.  C.  This  sketch  has  several 
significant  features:  it  shows  remarkable  uniformity  of  behavior  in  the 
four  test  panels;  it  indicates  high  stresses  along  inner  panel  edges,  as 
well  as  in  portions  of  the  unloaded  slab  along  the  outer  panel  edges;  and 
it  shows  several  important  cracks  around  the  center  column.  It  is  seen 
that  one  crack  formed  just  outside  of  the  inner  ring  of  Unit  C along  its 
entire  circumference.  This  ring  is  located  3 in.  outside  of  the  edge  of  the 
column  capital.  Another  crack  w^as  found  just  outside  the  second  ring  for 
a part  of  its  circumference.  A third  series  of  large  cracks  was  found  just 
above  or  a few  inches  outside  of  the  edge  of  the  drop,  and  radial  cracks 


FIG.  7.  TYPICAL  LOAD-STRAIN  DIAGRAMS. 


extended  outward  from  just  above  the  four  corners  of  the  drop  for  a 
considerable  distance  toward  the  center  of  the  test  panels. 

It  should  be  noted  that  many  of  the  cracks  were  found  near  and  fre- 
quently across  tension  gage  lines,  and  this  explains  some  variations  in 
stresses  measured  on  gage  lines  having  similar  locations  on  the  slab. 

Further  information  regarding  the  formation  of  cracks  is  suggested  by 
the  load-stress  curves  of  Fig.  7,  which  were  plotted  from  the  strain  meas- 
urements. Such  diagrams  for  gage  lines  lying  across  or  near  the  sections 
of  maximum  moment  show  a bend  or  change  in  slope  in  the  curves  at  a 
point  between  the  loads  of  200  and  350  lb.  per  sq.  in.  This  is  true  for  the 
tensile  stresses  on  both  upper  and  lower  sides  of  the  slab.  The  bend  occurs 
at  a measured  stress  of  about  3000  to  5000  lb.  per  sq.  in.  Cracks  were 
generally  visible  at  these  gage  lines  at  the  load  of  350  lb.  per  sq.  ft.,  and 
their  development  quite  evidently  produced  the  increased  rate  of  stressing 


Flat-Slab  Floor  Test. 


11 


of  the  steel.  At  points  of  less  stress  the  cracks  developed  later.  At  the 
maximum  load  the  largest  cracks  on  the  lower  side  of  the  slab  were  ob- 
served at  the  centers  of  panels  and  were  estimated  to  be  in  the  neighbor- 
hood of  0.01  in.  in  width.  At  the  same  load,  the  cracks  on  the  upper  side, 
which  ran  from  column  to  column,  were  estimated  to  be  0.015  in.  in  width. 
Around  the  column  head  and  at  places  just  outside  of  the  drop  at  the 
center  column,  the  estimated  width  of  the  cracks  was  0.015  to  0.02  in. 
The  other  cracks  noted  were  smaller.  It  should  be  noted  that  all  cracks 
closed  up  very  well  upon  removal  of  the  load. 


Effect  of  Continued  Loading. 

After  the  maximum  load  of  650  lb.  per  sq.  ft.  had  remained  in  place 
18  hours  an  increment  of  150  lb.  per  sq.  ft.  was  removed,  and  the  remain- 
ing load  of  500  lb.  per  sq.  ft.  was  left  undisturbed  over  nearly  the  whole 
area  for  a period  of  7 days.  At  the  end  of  this  time  a complete  set  of 
strain  and  deflection  readings  was  taken.  These  measurements  showed 
that  the  stresses  in  the  steel  were  about  90  per  cent,  and  the  strains  in  the 
concrete  were  about  100  per  cent  of  the  corresponding  stresses  and  strains 
under  the  load  of  650  lb.  per  sq.  in.  The  measured  deflections  were  about 
90  per  cent  as  great  as  those  measured  under  the  maximum  load.  Since 
the  continuously  applied  load  was  only  77  per  cent  of  the  maximum  load, 
it  follows  that  the  measured  strains  were  from  17  to  30  per  cent  greater 
than  would  be  expected  from  the  ratio  of  the  two  loads. 

The  load  of  500  lb.  per  sq.  ft.  was  gradually  removed  from  the  floor 
over  a period  of  about  two  weeks,  after  which  another  set  of  strain  and 
deflection  readings  was  taken.  The  recovery  of  strain  in  the  reinforcement 
was  found  to  be  from  50  to  60  per  cent,  leaving  40  to  50  per  cent  of  the 
maximum  strain  remaining  in  the  steel  after  the  test  load  had  been  re- 
moved. The  residual  strains  in  the  concrete  were  still  greater,  being  about 
70  per  cent  of  those  measured  at  the  maximum  load.  The  average  recovery 
in  deflection  for  all  observation  points  was  about  50  per  cent  of  the  max- 
imum deflection.  The  recovery  in  deflection  is  shown  in  Fig.  5. 

While  the  proportion  of  recovery  seems  rather  low  for  this  slab,  it 
must  be  remembered  that  the  test  load  remained  on  the  floor  longer  than 
is  usual  in  building  tests  and  that  the  concrete  was  only  about  two  months 
old  at  the  time  of  the  test.  A considerable  amount  of  plastic  deformations 
is  to  be  expected  under  these  conditions. 


Measured  Stresses  and  Deformations. 

The  measured  stresses  and  deformations  have  been  used  to  plot  load- 
stress  and  load-strain  diagrams,  a few  of  which  are  presented  in  Fig.  7. 
These  curves  are  useful  principally  in  showing  the  relative  magnitude  of 
stresses  and  strains  measured  at  the  different  increments  of  load. 

It  is  thought  best  to  confine  the  following  discussion  mainly  to  stresses 


w / / >VXo)<vA  \ I i ! \f]  n '\  / 4fi'\\\ 


! (X 


>.00053  | 


h"|ii 1 '•  g 


'ill  'vVvW Jit  /’IMMCTCT7 

A Vw-ft- AX/ 


/ A / V \ V \A  /n / y V 


16600  - 


\s]  tt/  v\x  , AAA  XV  > L-U; xN 

II  *,««*«  .MW  / ,7/7vX/\  \ \ 

I | /V^^W'  ! ! i'  ll  I \ §44*?°°-r 

W«»y/teferoni<sn  “Vi/ \ \ VX>^&XXvV  / / 'v  V )l  \ ' ' 

inrnr,m4~  Av~k  \ \AX>fx>s/V  / 


fS^.9400 


FIG.  8.  STRESSES  AND  DEFORMATIONS  ON  UPPER  SIDE  OF  SLAB  AT  MAXIMUM 

APPLIED  LOAD. 


6900 

J^l  / 
"^-+.+§800 
'^_~J75d0'4[  X 

id/OQ > K, 
-^rJ3900 


/«^ox  Xxx;<  - . 

-oxXHBaar;* 

x.  X 


c\Vjsa 

PI|iI^U$W+W+ 

Si  ^ 


inr 

o+oTooo^oX 


/-V  / /V\^v_X'v  y\  \ y v v\  / x'i  <^.00050^ 

n vXXfHX 


-V^ 


15000 

m$ 

+r  -±17200 

-Rk 

14600 

/IIJXIS 

^ 00034 
19100  r 
■y.00009 
19500 

'Y  1/^9000 
I8400  Y / 

X-A 

#200 


X ~ \^.d0006<^r*/5^vS< 
xX  \I560qXX  - 
X .00007- 


Symbols 


x\  / 

— 1 1 Unit  Stress  in  Steel  \ A / 
lb.  per  sg.  /n.  "I  y 
Tension  un/ess  marJh?d(-\ 


| 1 1 i \ » 

Unit  Deformation  V V \ \ 'CSi°y— 

in  Concrete  27\  V W - 

Compression  unless  mar/codfhk  A . 


/ /\-i/.  00020 
‘ ' 120600 


AUs  / / .000^5  - 

A7\  / pooozXp  vo§\  \ A §Tv  / xT>  \ \ 

IW* 


FIG.  9.  STRESSES  AND  DEFORMATIONS  ON  LOWER  SIDE  OF  SLAB  AT  MAXIMUM 

APPLIED  LOAD. 


14 


Flat-Slab  Floor  Test. 


at  the  maximum  applied  load,  since  at  the  lower  loads  there  is  a possibility 
of  greater  relative  error  and  since  the  stress  distribution  is  masked  to  a 
greater  degree  by  the  tension  in  the  concrete.  For  this  reason,  the  maxi- 
mum measured  stresses  and  deformations  at  the  load  of  650  lb.  per  sq.  in. 
are  presented  in  Figs.  8 and  9 for  the  upper  and  lower  sides  of  the  slab, 
respectively. 

It  will  be  noted  that  at  only  a few  gage  lines  did  the  stress  in  the 
reinforcement  exceed  25,000  lb.  per  sq.  in.,  the  highest  individual  stress 


FIG.  10.  COMPRESSIVE  UNIT  IN  SLAB  AROUND  CENTER  COLUMN. 


being  33,500  lb.  per  sq.  in.  The  average  of  the  stresses  measured  across 
the  principal  moment  sections  on  both  sides  of  the  floor  was  about  18,000 
lb.  per  sq.  in. 

The  compressive  strains  in  the  concrete  are  quite  uniformly  distributed 
along  the  sections  used  except  at  places  in  and  near  the  drop.  Fig.  10 
shows  measured  unit  deformations  in  the  concrete  of  the  lower  side  of  the 
slab  near  the  column  at  the  center  of  the  test  area.  Since  neither  the 
stress-strain  relation  for  the  concrete  nor  the  modulus  of  elasticity  of  the 
material  in  the  slab  is  known  accurately,  no  attempt  has  been  made  to 
calculate  compressive  stresses.  The  maximum  compressive  stresses  devel- 


Flat-Slab  Floor  Test. 


15 


Mid  Section 


| Colunyi-head5ection\ 


§ § Sit  § §saSgg 


5&S  3 3 

/r  m 


Upper  S/de  of  Slab 

Mid  Section  I Column-head  SectionX  Mid  Section  I 


Lower  S/de  of  Slab 


FIG.  11.  STRESSES  AND  DEFORMATIONS  ON  JOINT  COMMITTEE  SECTIONS. 


16 


Flat-Slab  Floor  Test. 


oped  appeared  to  be  generally  less  than  one-half  the  ultimate  strength  of 
the  concrete;  in  a few  cases  they  were  considerably  greater.  The  greatest 
individual  unit  deformation  measured  was  0.00079. 

It  is  seen  in  Fig.  10  that  the  deformation  in  the  floor  just  outside  the 
drop  is  quite  high,  as  it  is  also  in  the  diagonal  directions  on  the  under 
side  of  the  drop  just  next  to  the  column  capital.  A drop  of  this  deptli 
evidently  causes  very  abrupt  changes  in  compressive  stresses,  as  well 
as  regions  of  high  stress  in  the  reinforcement  above. 

The  stresses  and  deformations  along  sections  of  maximum  positive  and 
negative  moment  are  of  considerable  interest.  In  Fig.  11,  the  stresses  and 
deformations  along  the  standard  sections  designated  by  the  Joint  Com- 
mittee of  Concrete  and  Reinforced  Concrete*  have  been  plotted. 

On  the  upper  side  of  the  slab  the  highest  stresses  were  observed  in 
the  straight  reinforcing  rods  of  the  mid  section  (Unit  T ).  The  stresses 
in  the  columti  head  section,  particularly  those  within  the  drop,  were 
smaller.  The  unit  deformations  in  the  sections  of  positive  moment  do  not 
show  much  variation. 

On  the  lower  side  of  the  slab  the  stresses  in  the  reinforcement  are 
fairly  uniform  for  both  the  inner  and  outer  sections  of  positive  moment, 
except  at  the  ends  near  the  edge  of  the  loaded  area.  The  unit  deformations 
in  the  mid  section  of  negative  moment  are  quite  uniform,  while  the  de- 
formations in  the  column-head  section  show  a characteristic  variation  at 
all  gage  lines  near  the  edges  of  the  drop. 

It  may  be  noted  that  the  narrowness  of  the  bands  of  diagonal  bars 
(Unit  B,  truss  bars)  enables  the  reinforcement  to  act  to  good  advantage, 
since  the  bands  run  nearly  at  right  angles  to  the  sections  of  maximum 
moment  which  they  cut,  while  in  the  usual  four-way  system  the  bars  at 
the  edge  of  the  diagonal  bands  would  cross  the  panel  boundaries  at  points 
where  the  intensity  of  bending  moment  in  the  direction  of  the  panel  edge 
differs  greatly  from  that  at  right  angles  to  it. 

Stresses  Along  Rings. 

It  was  not  found  feasible  in  this  test  to  take  observation  on  con- 
secutive gage  lines  along  the  different  rings,  but  in  a number  of  cases 
several  gage  lines  were  located  on  a quadrant  of  a ring.  From  these 
observations  a good  idea  of  the  behavior  of  the  rings  in  various  parts  of 
the  slab  is  obtained.  Assuming  a uniform  variation  in  stress  between 
observation  points  the  stresses  in  a number  of  rings  at  a load  of  650  lb. 
per  sq.  ft.  are  shown  diagrammatically  in  Fig.  12. 

It  is  seen  that  the  rings  of  Unit  B show  maximum  tension  at  points 
where  they  cross  the  section  of  maximum  positive  moment  and  that  the 
tension  is  small  or  changed  to  compression  at  points  where  they  cross  the 
diagonal  of  the  panel.  The  compression  is  greater  in  the  larger  rings.  The 


♦Final  Report  of  the  Joint  Committee  on  Concrete  and  Reinforced  Concrete. 
July  1,  1910. 


Flat-Slab  Floor  Test. 


17 


LOCATION  OF  RIN65 


Assumed:  Uniform  variation 
in  stress  between  gage  tines 
Stresses  plotted  in  radial  di- 
rection. Load=  650 Ib  /w  5o.Fr 


FIG.  12.  STRESSES  ALONG  KINGS. 


18 


Flat-Slab  Floor  Test. 


inner  rings  of  the  unit  undoubtedly  have  more  nearly  uniform  stress 
throughout  their  length. 

The  stresses  in  the  rings  of  Unit  A are  seen  to  change  from  tension  to 
compression  in  a quadrant  of  the  ring.  Obviously,  the  sections  of  the  ring 
which  cut  the  outer  section  of  positive  moment  which  is  at  right  angles 
to  the  outer  section,  are  in  compression.  In  fact,  this  mid  section  was 
rather  highly  stressed,  as  is  shown  by  the  compressive  deformations  in  the 
concrete  and  the  high  tensile  stresses  in  the  top  rods  (Unit  T)  in  Fig.  11. 
Hence  it  seems  probable  that  with  so  great  a variation  in  stress  in  thesfc 
rings  the  stress  is  brought  into  the  bar  almost  wholly  by  bond  stress. 

The  action  of  the  rings  of  Unit  C and  of  the  adjacent  straight  bars 
is  more  difficult  to  analyze.  Fig.  12  shows  two  of  these  rings  to  be  in 
tension  throughout  their  length,  but  the  tension  is  greatest  at  the  panel 
edges  and  generally  less  at  the  panel  diagonals.  It  seems  clear  that 
changes  in  the  intensity  of  stress  in  a circular  ring  acted  upon  by  a uni- 
form internal  pressure  must  be  produced  by  bond  stress;  if  the  stress  is 
nearly  uniform  it  would  seem  to  be  developed  by  the  lateral  pressure  or 
bearing  exerted  against  the  inside  of  the  ring.  The  tension  in  the  rings 
of  Unit  G seems  to  be  developed  mainly  by  lateral  pressure. 

The  distribution  of  stresses  in  cantilever  flat  slabs  similar  to  the 
column-head  section  of  the  floor  tested  has  been  analyzed  mathematically 
by  a number  of  writers.  A simple  statement  of  the  principles  involved 
may  help  to  explain  the  relations  of  the  different  stresses  measured. 

Because  of  its  great  effective  depth,  the  portion  of  the  slab  over  the 
column  capital  does  not  develop  appreciable  deformations  under  an  ex- 
ternal moment.  Immediately  outside  the  capital,  however,  a considerable 
radial  deformation  occurs,  causing  a large  radial  unit  deformation.  A 
circular  ring  a small  distance  outside  the  column  capital  will  have  its 
diameter  increased  by  the  increment  of  radial  strain  outside  the  column 
capital,  and  its  unit  deformation  will  be  equal  to  the  radial  deformation 
on  the  two  sides  of  the  capital  divided  by  the  diameter  of  the  ring,  a 
comparatively  small  quantity.  It  is  seen  that  the  radial  unit  deformation 
is  large  at  the  edge  of  the  capital  and  decreases  with  the  bending  moment 
at  points  farther  away  from  the  capital.  The  unit  circumferential  deforma- 
tion, or  unit  deformation  in  a circular  ring,  however,  is  small  for  a ring 
near  the  column  capital  and  increases  as  the  diameter  of  the  ring  increases 
until  it  reaches  its  maximum  at  some  distance  from  the  edge  of  the  capital. 

A comparison  of  the  action  of  the  reinforcement  in  the  column-head 
section  is,  therefore,  difficult,  even  if  it  were  not  complicated  by  the  dif- 
ference in  effective  depth  of  bars,  the  difference  in  the  available  width  of 
drop  in  different  directions,  and  the  difference  in  the  manner  in  which  the 
stress  is  produced  in  the  reinforcing  bars.  It  is  evident  that  the  stresses 
are  developed  in  the  bars  of  the  rectangular  and  diagonal  bands  by  bond 
stress  in  the  usual  way.  In  the  rings  in  which  the  stresses  are  nearly 
uniform  the  stress  appears  to  be  developed  by  means  of  pressure  trans- 
mitted through  the  concrete  and  applied  as  bearing  pressure  against  the 


Flat-Slab  Floor  Test. 


19 


inner  side  of  the  ring.  This  action  involves  a shortening  of  the  width  of 
a ring  of  concrete  at  the  top  of  the  slab  just  inside  the  reinforcing  ring 
(on  the  tension  side  of  the  slab)  and  the  consequent  formation  of  a crack 
or  cracks  of  sqme  size  outside  the  next  smaller  ring.  The  variation  of 
stress  in  the  ring  and  in  the  straight  bars  at  and  near  where  they  cross 
the  ring  was  found  to  be  that  indicated  by  theoretical  consideration.  The 
greatest  stress  in  a direct  or  diagonal  bar  was  found  over  the  edge  of  the 
column  capital,  and  the  stress  decreased  outwardly,  except  as  influenced 
by  the  shape  and  size  of  the  drop.  The  smallest  two  rings  of  Unit  G did 
not  develop  their  share  of  stress,  and  it  appears  that  the  rings  of  this  unit 
are  most  effective  beyond  a distance  from  the  capital  equal  to  one  or  more 
times  the  thickness  at  the  drop. 

Distribution  of  Stresses  and  Moments. 

One  of  the  main  objects  of  the  test  was  to  see  how  effectively  the 
reinforcing  steel  was  distributed  throughout  the  slab  and  to  determine 
the  proportion  of  the  total  bending  moment  developed  at  the  various 
sections.  From  the  stresses  observed  the  distribution  of  the  steel  has  been 
shown  to  be  fairly  good;  in  a few  instances  a slight  rearrangement  of  the 
reinforcement  would  eliminate  the  extremes  of  very  high  and  very  low 
stresses.  Table  II  gives  average  values  of  stresses  in  the  reinforcement  at 
a load  of  650  lb.  per  sq.  ft.  as  measured  on  the  various  sections  shown  in 
Fig.  11. 

Regarding  the  resisting  moments  developed  by  the  steel,  as  has  always 
been  found  in  other  tests  of  reinforced  concrete  floors  at  stresses  which 
were  considerably  below  the  yield  point  of  the  steel,  the  measured  stress 
over  the  full-gage  length  does  not  account  for  the  full  analytical  value 
of  the  bending  moment  produced  by  the  load.  With  a crack  present  in  the 
concrete  across  a gage  line,  it  is  to  be  expected  that  the  average  unit  strain 
over  the  gage  length  will  be  less  than  the  unit  strain  over  some  part  of 
the  gage  length.  The  concrete  in  the  earlier  stages  of  loading  resists  a 
considerable  part  of  the  bending  moment.  Experience  in  other  tests  has 
shown  that  as  the  stress  in  the  reinforcing  bars  approaches  the  yield 
point,  the  reinforcement  gradually  takes  a greater  proportion  of  the  full 
bending  moment  and  finally  assumes  its  full  share.  It  is  evident  that  the 
tension  in  the  concrete  varies  with  the  percentage  of  reinforcement,  as 
well  as  with  the  quality  of  the  concrete.  Some  quantitative  data  on  this 
phenomenon  from  tests  of  a variety  of  structures  have  been  published 
recently  by  Prof.  Hatt.*  From  an  analysis  of  the  tests  of  a number  of 
buildings,  he  found  that  the  total  resisting  moment  of  the  steel  at  a 


*“ Moment  Coefficients  for  Flat  Slab  Design,  with  Results  of  a Test,”  by 
Prof.  W.  K.  Hatt,  Proc.  A.  C.  I.  1918. 


Table  II. 

Stress  and  Moment  Distribution  at  Load  of  648  lb.  per  sq.  ft.  (Dead  load  stresses  not  included.) 


Flat-Slab  Floor  Test. 


i 

"§  i00 


O O E- 


% 


Flat-Slab  Floor  Test. 


21 


measured  steel  stress  of  18,000  lb.  per  sq.  in.  was  about  40  to  50  per  cent 
of  the  full  theoretical  moment  of  4_cy  2 

wherein  c « = diameter  of  column  capital,  in  incites,  feet 
lo  Z — total  load  per  sq.  ft. 

Jl  g — panel  length,  in  inches. 

To  obtain  data  on  the  effect  of  the  tension  in  the  concrete,  two  test 
beams  were  poured  at  the  time  that  the  concrete  in  the  test  area  was 
poured.  However,  the  percentage  of  reinforcement  used,  which  was  .0071, 
compares  only  fairly  well  for  the  column-head  and  outer  sections  and  is 


FIG.  13.  RATIO  OF  MOMENTS  AT  VARYING  STRESSES  IN  STEEL. 


much  higher  than  the  percentage  for  the  inner  and  mid  sections  of  the 
test  slab.  Fig.  13  shows  the  ratio  of  the  resisting  moment  of  the  steel  to 
the  bending  moment  at  different  measured  stresses  in  the  two  beams.  The 
ratio  of  the  two  moments  at  a measured  stress  of  18,000  lb.  per  sq.  in. 
is  seen  to  be  about  0.72,  but  this  ratio  would  be  much  smaller  for  a per- 
centage equal  to  the  average  percentage  used  in  the  slab. 

From  the  average  stresses,  effective  depths  and  steel  areas,  the  resist- 
ing moments  at  the  various  design  sections  have  been  calculated  and  are 
given  in  Table  II.  A study  of  these  moments  shows  that  only  about 
36  per  cent  of  the  analytical  moment  is  accounted  for  by  the  steel  stresses 


22 


Flat-Slab  Floor  Test. 


at  the  load  considered.  The  distribution  of  the  moments,  however,  agrees 
fairly  well  with  the  distribution  commonly  used  in  design.  It  is  found  that 
the  negative  moment  is  60.7  per  cent,  and  the  positive  moment  is  39.3  per 
cent  of  the  total  amount,  and  that  the  further  subdivision  of  these 
moments  among  the  different  sections  also  agrees  very  well  with  the  usual 
assumptions  regarding  moment  distribution  in  flat  slab  floors. 

General  Comments. 

The  measured  stresses  at  points  other  than  the  standard  design  sec- 
tions did  not  appear  to  be  of  particular  importance,  except  that  high 
stresses  were  found  on  both  the  tension  and  compression  sides  near  the 
edge  of  the  dropped  panels. 

The  calculation  of  moments  at  intermediate  sections  was  difficult 
since  the  position  of  the  point  of  inflection  was  not  fully  known.  From 
the  data  available  it  appears  that  the  distance  from  the  panel  edge  to  the 
line  of  inflection  was  about  three-tenths  of  the  panel  length. 

The  slab  was  not  loaded  heavily  enough  to  develop  much  evidence  of 
the  action  of  shear  and  diagonal  tension.  It  has  been  questioned  whether 
a reinforcing  system  of  rings  provides  properly  against  shearing  failure 
which  might  occur  at  a circular  section  near  one  of  the  rings  around  the 
column  head  at  some  distance  outside  the  column  capital.  It  was  found 
in  the  test  that  tracks  followed  such  a section  along  rings  of  Unit  C. 
Whilecracks  of  this  soi;t  are  also  found  in  slabs  having  two-way  or  four- 
way reinforcement,  with  the  ring  reinforcement  the  cracks  are  likely  to 
be  wider  and  to  be  concentrated  upon  a definite  path. 

It  is  felt  that  the  building  withstood  the  test  very  well.  The  stresses 
are,  if  anything,  lower  than  might  have  been  expected  at  the  applied 
load,  and  are,  with  a few  exceptions,  fairly  uniform.  It  is  hoped  that  the 
results  of  this  test  will  add  considerably  to  the  knowledge  of  the  behavior 
of  this  system  of  flat-slab  reinforcement. 

The  writers  wish  to  acknowledge  the  hearty  and  efficient  co-operation 
of  It.  F.  Wilson  & Co.,  who  had  charge  of  applying  the  test  load;  of  the 
Chicago  Building  Department,  and  of  the  Structural  Research  Laboratory 
at  Lewis  Institute.  Acknowledgment  is  also  made  t<?  Professor  Talbot  for 
assistance  in  the  planning  and  performance  of  the  test  and  for  helpful 
advice  in  the  preparation  of  this  paper. 


